1) Field of the Invention
The present invention relates to a semiconductor photo detector for use in optical measurements or optical communications.
2) Related Art
Currently, optical fibers are widely used in cables of optical communications; in order to maintain and manage such optical fiber networks, an optical time domain reflectometory method (hereinafter referred to as "OTDR"), is used for finding out damage of the optical fibers. The basic principle thereof is: a pulse of light is made incident into the optical fiber and is transmitted therethrough whereby a Rayleigh scattered light is generated; any damage of the optical fiber can be found out by monitoring the thus generated Rayleigh scattered light. That is to say, if the optical fiber has some damage, the Rayleigh scattered light might not be returned to the monitoring point, so that the point of damage on the optical fiber can be correctly obtained by converting the time period from when the monitoring starts until the time when the Rayleigh scattered light cannot be monitored into a distance.
In such a system, it is necessary to connect the light communication line to the OTDR, and therefore, the communication line is temporarily interrupted. In order to solve this drawback, it is considered to conduct monitoring at all times while using the light communication line as it is. In this case, the monitoring is conducted by using light having a wave length of 1.65 .mu.m, which is different from the wavelength of transmission light used in the light communications, i.e. 1.3 .mu.m or 1.55 .mu.m.
An Avalanche Photo Diode (hereinafter referred to as "APD") is used as a semiconductor photo detector for use in such a monitoring system. Since the APD per as has its own amplifying function, it is generally used as a photo detector having a high reliability in the fields of optical measurements or optical communications. Particularly, lnP/InGaAs are widely used as materials in semiconductor photo detectors for use in long distance and large capacity optical communication systems, where light having a wavelength of 1.3 .mu.m or 1.55 .mu.m is used.
FIG. 1 is a cross sectional view showing a construction of the conventional type of APD in which InP/InGaAs is used as light absorption layer. As shown in FIG. 1, the APD has its construction such that on a substrate 1, which is made of n+ type InP, an epitaxial crystal layer is formed, where a buffer layer 2 made of n type InP having a carrier concentration of 1E14 to 1E16 cm.sup.-3 and a thickness of 1 to 3 .mu.m, a light absorption layer 15 made of n- type In.sub.0.53 Ga.sub.0.47 AS having a carrier concentration of 1E14 to 1E16 cm.sup.-3 and a thickness of 1 to 5.mu.m, an intermediate layer 4 made of n type InGaAS having a carrier concentration of 1E15 to 1E16 cm.sup.-3 and a thickness of 0.3 to 1 .mu.m, and a multiplication layer 5 made of n+ type InP having a carrier concentration 2E16 to 4E16 cm.sup.-3 and a thickness of 0.8 to 4 .mu.m are subsequently formed in this order by using a gas phase crystal growth method. On the epitaxial layer, a light receiving portion 8 made of p+ type InP layer having its carrier concentration of 1E17 to 1E20 cm.sup.-3 is selectively formed by a diffusion of Zn in a closed tube, and a guard ring 7 is further provided so as to surround the light receiving portion 8 by an ion implantation of Be.
When a reverse bias is applied to the InGaAs APD, a depletion layer is generated in the InGaAs light absorption layer 15. In case that light having a wave length of 1.3 .mu.m, which is less than a wave length of 1.67 .mu.m, corresponding to a band gap energy of the InGaAs light absorption layer 15, is made incident into the APD, carriers are generated in the depletion layer of the light absorption layer 15 by a photoelectric effect. The thus generated carriers are accelerated by an internal electric field of 20 to 100 kV/cm in the depletion layer to its saturated velocity, so that the carriers go out to an external circuit as an electric current.
In this system, however, there is a drawback as follows. That is to say, the light absorption layer 15 is made of n- type InGaAs, whose wavelength at its absorption edge is 1.67 .mu.m. Such a light having a wave length 1.67 .mu.m at its absorption edge does not cause any problem to the light having a wavelength of 1.3 .mu.m or 1.55 .mu.m, which is used for transmitting of information in the optical communication. Since the light having a wave length of 1.65 .mu.m, which is used for monitoring the Rayleigh scattered light, is close to the wavelength at the absorption edge of the light absorption layer 15, the sensitivity of the light used for the monitoring becomes low and the quantum effectivity thereof becomes about 20%.
FIG. 2 is a cross section showing the construction of another type of APD. The APD has its construction that on a substrate 1 made of n+ type InP are grown a buffer layer 2 made of n type InP and a crystal layer. The crystal layer is composed of a superlattice light absorption layer 16 made of n- type InAs/GaAs having a thickness of 1.5 to 2 .mu.m, a layer 4 made of n- type InGaAs having a thickness of 0.1 .mu.m, a multiplication layer 5 made of n+ type InP having a thickness of 1.5 .mu.m, and a layer 8 made of p+ type InP having a thickness of 1 .mu.m, in this order. On the surface of the crystal layer, a mesa etching is applied; a p type side electrode made of TiPtAu is provided and an n type side electrode made of AuGeNi to complete the photo detector. According to this APD, it is possible to make the wavelength at the absorption edge of the light absorption layer greater to 3.2 .mu.m, since a superlattice layer structure of InAs and GaAs is used.
However, this ADP still has a drawback in that since the superlattice structure of InAs and GaAs is formed on InP, a lattice mismatching is generated on the phase boundary therebetween, so that a dark current is generated by the lattice imperfection; since the dark current causes noise, the sensitivity of the monitoring becomes low.